Frequency hopping is a spread spectrum technique used in many radio-communication applications. In a frequency hopping spread spectrum system, the transmitter changes the frequency of its transmissions over time according to a pseudorandom hopping pattern. In effect, the transmitter “hops” from one frequency to another during transmission to spread its signal over a wide frequency band, while at any given moment, the transmitted signal occupies a narrow frequency band. The hopping period, referred to herein as a time slot, is the time interval during which the frequency remains constant. The frequency hopping pattern comprises the sequence of frequencies over which the transmitter hops.
Frequency hopping provides frequency diversity, which helps mitigate the effects of multipath fading provided that the spacing between sub-carriers is sufficiently large so that fading is uncorrelated across the different frequencies. Most mobile-communication systems apply channel coding at the transmitter side and corresponding channel decoding at the receiver side. To take advantage of the frequency diversity provided by frequency hopping, a block of coded information should be spread out over multiple hops, i.e. multiple time slots.
Frequency hopping may be used to share a radio resource between multiple users. In conventional frequency hopping systems, different mobile terminals within the same cell or sector of a mobile communication system are assigned mutually orthogonal frequency hopping patterns so that the mobile devices will not transmit simultaneously on the same frequency in the same time slot. One way to ensure that the hopping patterns are mutually orthogonal is to use the same basic hopping pattern for all mobile devices with different frequency offsets for each mobile terminal.
Between cells, different non-orthogonal frequency-hopping patterns are typically used, implying that simultaneous transmissions from two mobile devices in neighboring cells in the same frequency band during the same time slot may take place. When this happens, a “collision” occurs, implying a high interference level during the corresponding time slot. However, due to the channel coding spanning several hops, the channel decoder can typically still decode the information correctly.
Frequency hopping may be applied in Orthogonal Frequency Division Multiplexing (OFDM) systems. In OFDM systems, a wideband carrier is divided into a plurality of subcarriers. A Fast Fourier Transform is applied to the modulation symbols to spread the modulation symbols over multiple subcarriers of the wideband carrier. Frequency hopping may be implemented in OFDM systems by varying the subcarrier assignments.
Recently, there has been interest in using variable bandwidth allocations in the uplink of OFDM systems. The basic concept is to vary the bandwidth assigned to mobile terminals based on their instantaneous channel conditions, buffer level, Quality of Service (QoS) requirements, and other factors. A scheduler in the network schedules the mobile terminals and determines their bandwidth allocations.
Frequency hopping has not previously been used in OFDM systems that employ variable bandwidth allocation. One difficulty in applying frequency hopping techniques to an OFDM system that allows variable bandwidth allocations is that the number of available hopping patterns changes depending on the bandwidth allocations. Furthermore, when mixing transmissions from two or more mobile devices using different bandwidths within one subframe (FDMA), the hopping possibilities for each mobile device depends on the bandwidth allocated to the other mobile devices. Another problem is that bandwidth allocations are dependent on the instantaneous channel conditions of the mobile devices and thus cannot be known in advance. If the frequency pattern is establish without consideration of the bandwidth allocations, the bandwidth allocations must be made to avoid collisions, which will reduce the efficiency of the system.
Accordingly, there is a need for new scheduling techniques to enable frequency hopping in OFDM systems that allow variable bandwidth allocations.